Sn-induced surface modification of EUV plasma-facing collector mirrors

Generation of extreme ultraviolet (EUV) light for nanolithography requires the use of hot, dense plasmas. Gas discharge produced plasma (GDPP) and laser produced plasma (LPP) are the primary configurations used to generate these plasmas, The in-band EUV spectrum composes about 1% of the total light generated and efficient collection requires the use of thin-film grazing incidence mirrors or near-normal incidence multi-layer mirrors for GDPP and LPP, respectively. Both GDPP and LPP collector mirror systems face serious issues regarding lifetime and EUV light reflectivity performance. For both configurations debris, fast ions, fast neutrals, and condensable EUV radiator fuels (Li, Sn) interact with nearby collector mirrors, Sn, in particular is used due to its high conversion efficiency in the 13.5-nm in-band spectrum. In addition to debris, collector mirrors are exposed to impurities (H,C,O,N), off-band radiation (depositing heat) and highly-charged ions leading to their degradation and consequently limiting 13.5 nm light reflection intensity. This work presents results of Ru and Rh-based collector optics irradiated by Sn ions at 1.3 keV and exposed to Sn thermal atoms at room temperature. In-situ metrology monitors the effects of ion implantation on ion-induced erosion and EUV reflectivity changes using low-energy ion scattering spectroscopy and X-ray photoelectron spectroscopy. Results find that Sn implants at the first few monolayers leading to a mixed layer near the surface. Sn thermal atoms deposit on top of the mirror surface leading to 10-20% loss of collector mirror EUV light reflectivity. © 2006 Materials Research Society.